The invention pertains generally to the field of electronic transistors, and more particularly to heterostructure transistors operating by transfer of electrons across an interface between different semiconductor materials.
Heterostructure semiconductor devices have enjoyed considerable interest in the last several years as candidates for high-speed switches and for transistors that can generate and amplify high frequency electronic signals. One class of these devices operates on the principle of transfer of hot electrons between two different semiconductor layers separated by a potential barrier, where the temperature of the electrons is controlled by an applied electric field. Because the temperature of these electrons responds very rapidly to the applied field, and because the transit time of these electrons can be very short, such devices offer the promise of very high frequency operation.
These heterostructure devices have evolved from the well-known Gunn diode, which is a monolithic semiconductor structure that also employs a "transferred electron" mechanism. In the Gunn diode, a semiconductor material has a filled valence band and a partially filled conduction band. This conduction band has a central minimum-energy valley of electron states that are occupied up to the Fermi level, and one or more "satellite-valleys" at higher energies that are normally unoccupied. An example of such a material is GaAs, where the satellite valley minimum is 360 meV above the central valley minimum. If the GaAs is subjected to an electric field sufficiently large to give some of the conduction electrons more energy than this energy gap, they can be transferred into, and remain in, the satellite valley. Being in a different region of quasi-momentum space (or "k-space"), these satellite valley electrons have a different effective mass, which in GaAs is about 20 times higher than that in the central valley. The corresponding electron mobility is much smaller than that of the central valley electron states. The conductivity of the electrons is therefore reduced by the field, and the material exhibits a negative differential resistance. Semiconductor materials exhibiting such nonlinear behavior can be used to construct electronic oscillators, amplifiers, and switching and storage devices.
The heterostructure devices of interest comprise adjacent layers of different semiconductor materials in which the conduction bands of the materials have different mobilities. Instead of transferring electrons into lower mobility states in k-space, the applied electric field causes the conduction electrons in a high-mobility material to become heated and transferred in "real space" into an adjacent material of lower mobility. This means, again, that the conductivity of these electrons is effectively reduced by the applied field, and these heterostructures also exhibit negative differential resistance.
Negative resistance heterostructures have been disclosed in U.S. Pat. No. 4,257,055 (Hess et al.), and are described also in the following articles: "Negative Differential Resistance Through Real-Space Electron Transfer", K. Hess, H. Morkoc, H. Shichijo and B. G. Streetman, Appl. Phys. Lett. 35 (6), Sept. 15, 1979; "Measurements of Hot-Electron Conduction and Real-Space Transfer in GaAs-Al.sub.x Ga.sub.1-x Heterojunction Layers", M. Keever, H. Shichijo, K. Hess, S. Banerjee, L. Witkowski, H. Morkoc, and B. G. Streetman, Appl. Phys. Lett. 38 (1), Jan. 1, 1981; "Fast Switching and Storage in GaAs-Al.sub.x Ga.sub.1-x Heterojunction Layers", M. Keever, K. Hess and M. Ludowise, IEEE Electron Device Letters, Vol. EDL-3, No. 10, October 1982. These references all describe semiconductor heterostructures having adjacent alternate layers of high-mobility GaAs and low-mobility Al.sub.x Ga.sub.1-x As. Conduction electrons are supplied by donors in the Al.sub.x Ga.sub.1-x As and migrate to the GaAs layer, which has a smaller band gap and lower scattering. When an electric field is applied in a direction parallel to the interface between the layers, the electrons in the conduction band of the GaAs will be rapidly heated to energies well above their thermal equilibrium value, and acquire sufficient energy to overcome the conduction band discontinuity between the two materials and move back into the Al.sub.x Ga.sub.1-x As layer. This propagation can be described as a kind of thermionic emission. Once these electrons reach the low-mobility layer their conductivity is reduced and the heterostructure displays the negative differential resistance described above.
The above references describe the basic principles of real-space electron transfer and their application to switching and storage devices. These principles are well understood for the most part, although some of the details have yet to be elucidated, such as the precise physical location at which the electrons transfer. Interest has been growing in the application of these principles in order to design transistors that can amplify and generate high frequency signals.
One transistor design based on these principles is described in the article "Charge Injection Transistor Based on Real-Space Hot-Electron Transfer", S. Luryi, A. Katalsky, A. Gossard and R. Hendel, IEEE Transactions on Electron Devices, Vol. ED-31, No. 6, June 1984. This reference describes a transistor having a conducting n-GaAs substrate that acts as the collector of the device. Over this substrate is grown an undoped Al.sub.x Ga.sub.1-x As barrier layer, then an undoped GaAs channel layer, and a layer of n-Al.sub.0.34 Ga.sub.0.66 As which provides a source of electrons, capped by an n-GaAs layer. A source electrode (serving as the "emitter") and a drain electrode ("base") extend downward through the top three layers into, but not through, the barrier layer.
A gate electrode is also provided, extending through the cap into the electron source layer. This gate allows the device to function in a negative resistance field effect transistor mode (NERFET), as well as a charge injection transistor mode (CHINT). In the charge injection mode, which extends to higher frequency operation, this gate is not used and will not be discussed further here.
In the CHINT mode of operation with a common base (drain) configuration, positive voltages (with respect to the drain) are applied to the source and collector electrodes. The source-drain field causes an electron flow in the channel layer and raises the temperature of these electrons. They are normally confined to the channel layer, until the temperature becomes sufficiently high that they can overcome the barrier layer. The hot electrons are thereby injected by thermionic emission into the barrier layer region and drift to the substrate collector. As the drain voltage increases, the drain current initially rises, and then decreases as the electron temperature exceeds the barrier height and the collector current rises.
The transistor structure described in this Luryi reference has several limitations. One problem arises from parasitic leakage, i.e. direct injection of electrons from the source electrode into the collector. Another limitation arises from the delay time caused by the electron time of flight in the channel and barrier. A third limitation arises from the large capacitance between the substrate collector and the source (or drain). All of these limitations adversely affect the high frequency performance of this device. An improved version of this transistor structure is described in the article "High-Frequency Amplification and Generation in Charge Injection Devices", A. Katalsky, J. H. Abeles, R. Bhat, W. K. Chan, and M. A. Koza, Appl. Phys. Lett. 48 (1), Jan. 6, 1986. These references report measurements on CHINT devices indicating power gain up to 9.8 GHz and extrapolated current gain up to 29 GHz. These appear to be the upper frequency limits for CHINT devices attained to date.